1. Field of the Invention
This invention relates to an electrochemical cell having a molten salt electrolyte, particularly a cell which also has an active metal electrode therein.
2. The Prior Art
Present day electrochemical cells which employ molten salt electrolytes, operate at temperatures from 200.degree. C. to 600.degree. C. Such high temperatures cause material failures due to the harsh environment. Such batteries also require large amounts of insulation and complex thermal controls.
In the recent past, attention has been directed to reduced temperature molten salts. Thus chloroaluminate molten salts are mixtures of aluminum chloride and a chloride donor, which is usually an alkali metal chloride or organic chloride. The most studied chloroaluminate molten salts are the mixtures of NaCl/AlCl.sub.3, BPC/AlCl.sub.3 and MEIC/AlCl.sub.3, where BPC is 1-(1-butyl) pyridinium chloride and MEIC is 1-methyl-3-ethylimidzolium chloride. The latter two binary molten salts have the advantage of having compositions with melting points below room temperature, i.e. they are room temperature molten salts or ionic liquids.
In one example, 1-methyl-3-ethylimidazolium chloride and aluminum chloride, herein MEIC-AlCl.sub.3, many of the physical, chemical and electrochemical properties of the molten salt, depend on the composition, i.e. the relative proportions of MEIC and AlCl.sub.3. The maximum voltage that a battery cell can deliver is determined by the decomposition potentials of the electrolyte. The difference between the anodic and cathodic decomposition limits is called the electrochemical window of the electrolyte.
The window in the MEIC-AlCl.sub.3 melts depends on the chemical species present, which is determined by the composition thereof. The value for the molten salts is about 2.4V, compared with about 1.5V for water-based electrolytes. While 2.4V is a good window, the MEIC-AlCl.sub.3 molten salts have a particular composition where the window expands to 4.5V.
However, such wide window point, e.g. where MEIC and AlCl.sub.3 are equal, at 50 mole fraction each (called the neutral point) had been difficult to maintain until a recent invention which is disclosed in a patent application entitled; Method And Composition For Chloroaluminate Molten Salts Having A Wide Electrochemical Window, issued as U.S. Pat. No. 5,096,789 on Mar. 17, 1992, by J. S. Wilkes et al., which disclosure is incorporated herein by reference.
That invention relates to a method for obtaining and maintaining a wide electrochemical window in chloroaluminate molten salts by adding to such molten salts, (where mole fraction AlCl.sub.3 :MEIC is greater than or equal to 1:1), an alkali metal chloride salt, e.g. NaCl, to buffer such molten salts to a Lewis acid-base neutrality.
The present invention makes use of such reduced temperature (wide window) molten salts in constructing a reduced temperature electrochemical cell of relatively high voltage. That is, the applicant employs an active metal as the anode in a battery cell that contains the above buffered molten salt electrolye, as more fully discussed below.
In related prior art, U.S. Pat. No. 4,463,071 to Gifford et al (1984), relates to the use of chloroaluminate molten salts as electrolytes with the addition of alkali metal salts, including NaCl, to such electrolytes. However the Gifford reference discloses such salts as a source of active metal ion for insertion into their electrodes during cell operation (Col. 2, lines 18-21 and Col. 6, lines 15 and 16). That is, Gifford's anode will not function without such cation supply.
The present invention, however, adds alkali metal chloride to choroaluminate molten salts for a different purpose, to buffer them into wide window neutrality, as discussed above. Further, an anode of active metal is employed in the battery cell of the present invention.
In other prior art, U.S. Pat. No. 4,463,072, to Gifford et al (1984) and U.S. Pat. No. 4,882,244, to Donahue et al. (1989), disclose metal anodes in acidic electrolytes. In such electrolytes active metals chemically react to the point of distraction and are thus not suitable.
Accordingly, there has now been discovered a battery cell that makes what is believed the first use of an active metal as the anode therein, which contains a reduced-temperature molten salt electrolyte. The use of such active metal anode results in a higher voltage at, e.g. room temperature, than for any other known cell containing a molten salt electrolyte.